The field of the invention is ultrasonic transducers which radiate ultrasonic waves into the body of a patient and which receive and detect ultrasonic waves emanating from the body of a patient.
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements which are sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage is applied, the piezoelectric elements change in size at a frequency corresponding to that of the applied voltage. When a voltage pulse having an ultrasonic frequency is applied, the piezoelectric element emits an ultrasonic wave in the media to which it is coupled. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic matching layer that improves the coupling with the media in which the ultrasonic waves propagate. In addition, a backing material is disposed to the rear of the piezoelectric element to absorb ultrasonic waves that emerge from the back side of the element so that they do not interfere.
When used for ultrasound tomography, the transducer has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the phase of the applied voltages, the ultrasonic waves produced by the piezoelectric elements combine to produce a net ultrasonic wave which is focused at a selected point. By controlling the phase of the applied voltages, this focal point can be moved in an azimuthal plane to scan the subject. However, objects which are not at the focal plane which is orthogonal to the azimuthal plane and parallel to the surface of the array are out of focus and their resolution in the reconstructed image is reduced. Thus, ultrasonic transducers focus the wave providing very high resolution images of objects lying at or near the focal plane, but have increasingly lower resolution of objects lying to either side of this plane. Such transducers are said to have high resolution, but low depth of field.
In very high quality medical imaging equipment ultrasonic transducers having an array of annular shaped piezoelectric elements have been used. Such prior transducers are driven by Gaussian shaded or Fresnel shaped voltages to provide high resolution within a relatively shallow depth of field. Outside the depth of field the resolution degrades due to diffraction effects.
Nondiffracting solutions to the wave equation governing their propagation (the scalar Helmholtz equation) have recently been discovered and extensively tested with electromagnetic waves. This solution was described by J. Durnin in an article "Exact Solutions for Nondiffracting Beams. I. The Scalar Theory." published in the Journal of Optical Society of America 4(4):651-654, in April, 1987. This solution indicates that transducers can be constructed which produce a wave that is confined to a beam that does not diffract, or spread, over a long distance. Such a nondiffractive beam can produce a much greater depth of field than a focused Gaussian beam.